In order to produce a print using electrophotographic means, a primary imaging member, also referred to as a photoreceptor or a photoconductor, is first uniformly charged. An electrostatic latent image is then formed by image-wise exposing the charged using known methods such as an optical exposure system, an LED array, or a laser scanner. The primary imaging member is then brought into close proximity to a development station that contains electrically charged marking particles, often referred to as toner or dry ink so that the marking particles selectively adhere to the electrostatic latent image, thereby converting it into a visible image. The image is then transferred to an intermediate transfer member or directly to a receiver. The visible image on the receiver is then made permanent by fixing or fusing the image by, for example, subjecting the image-bearing receiver to a combination of heat and pressure. If desired, a gloss can be imparted onto the image by casting the image against a ferrotyping member, as is known in the art. The primary imaging member is then cleaned and made ready to produce a subsequent print.
To produce a color image, electrostatic latent images are formed corresponding to the subtractive primary colorant information, i.e. the cyan, magenta, yellow, and black colors of that comprise the color gamut of the image to be printed. These images, frequently referred to as separations, are transferred in register either to a receiver directly or to an intermediate member and then to the receiver. The image is then fixed, as described above.
Two distinct electrophotographic engine designs are used to produce color images using an electrophotographic module to produce an electrophotographic image. The first is an electrophotographic module that contains a primary imaging member, a primary charger, a means for creating an electrostatic latent image, a means for converting the electrostatic latent image into a visible image, and a means for transferring the visible image to either a transfer intermediate member or a receiver. The electrophotographic module can also contain appropriate cleaning devices or means to remove residual toner, etc. where necessary and appropriate. The printer also has other components not in the electrophotographic modules, such as a fuser, a receiver or paper feeding device, finishing devices such as staplers, stackers, collators, etc. In some printers there are also intermodule components such as a paper inverter.
In some instances components can be shared by more than one module. For example, a single primary imaging member in the form of a web can be used to create the electrostatic latent image corresponding to each of the separations. In this example, it is generally preferred to have different frames of the web primary imaging member used for each separation and then transfer the separations sequentially and in register to either an intermediate member or to a receiver. It is generally not desirable to use a cylindrical primary imaging member as a shared component in multiple electrophotographic modules as the size of such a cylinder would be prohibitively large and expensive.
Currently any electrophotographic engines with a plurality of development stations located in proximity to a single primary imaging member produce color prints by serially developing electrostatic latent images onto a primary imaging member. These engines require that a toned image be first transferred from the primary imaging member prior to the formation of a subsequent electrostatic latent image and the conversion of the electrostatic latent image into a visible image or the polarity of the charge of the toner in the two stations be opposite. In either application, converting the electrostatic latent image to a multiple of images with differing colors or toners requires that the sequential charging, formation of the electrostatic latent image, and conversion of the electrostatic latent image into a visible image. Thus, an image requiring two colors takes at least twice as long as that described in the present invention. For example an image requiring four colors would take four times as long to produce as one utilizing a single color.
Also present electrophotographic printers have other limitations. The color gamut obtainable is limited to that area in color space spanned by the subtractive primary colored toners. Thus, colors that contain vivid reds or greens might not be printable. Green, red, orange, blue, and violet toners are specialty toners that are used to enhance the available color gamut. Custom spot colors such as are commonly used in corporate logos are often outside the realm of the color gamut obtainable with standard subtractive primary colors. Magnetic recording inks, called MICR toners, are often used by banks to mark checks. These generally require a separate development station. The density versus the log of the exposure, often referred to as the D-logE curve, tends to become flat in both the low and high density regions of a print. These regions are referred to as the toe and shoulder, respectively, and are accompanied by a loss of information. It simply is not possible to differentially deposit varying amounts of toner in these regions to enhance the information. However, amounts of toner having a lower than normal colorant density or extinction coefficient can enhance the information content of these regions.
Another example of the limitation of the present technology is that there are many types of specialty toner required for one print. For example, normal-size clear toner particles, i.e. those having median volume weighted diameters in the range of approximately 5 μm to 8 μm, are often used to cover exposed portions of a receiver such as paper to enable an image on that receiver to be uniformly glossed and large clear toner, i.e. that having a median volume-weighted diameter of greater than approximately 20 μm, is often used to produce raised letter printing. In addition, toner particles are often used that contain security features that might be desired in the print, such as toner particles can contain so-called traceless components that would allow only certain detectors to detect the presence of the component. Combining all these toners, called specialty toners, into one latent images is not currently possible in one pass since in most electrophotographic print engines, the receiver is in sheet format. Transporting a sheet through a large number of electrophotographic modules is problematic and can lead to misregistration as well as artifacts such as fuser oil being transported back to a sheet from a transport web. As the length of the web increases to allow for additional electrophotographic modules, the probability of back transferring fuser oil from the transport web to the receiver increases.
In addition, these toner particles are often highly charged electrically. If there are too many toner particles present, such often occurs when multiple layers of toner are present, the electrostatic field used to transfer the toner is screened by the toner charge, thereby reducing the transfer field and impeding transfer. Thus, it is often difficult to transfer an arbitrarily large number of toner layers, in contrast to the lithographic printing of an arbitrarily large number of offset printed separations.
Finally, the space available for electrophotographic print engines is generally much more restricted than that available for offset presses.
The present invention allows all the printing of these specialty toners into one printer using one or more multi-development status.